1. Field of the Invention
The present invention relates to optical substrates having a structured surface, particularly to optical substrates for brightness enhancement, and more particularly to brightness enhancement substrates for use in flat panel displays having a planar light source.
2. Description of Related Art
Flat panel display technology is commonly used in television displays, computer displays, and handheld electronics (e.g., cellular phones, personal digital assistants (PDAs), etc.). Liquid crystal display (LCD) is a type of flat panel display, which deploys a liquid crystal (LC) module having an array of pixels to render an image. In backlight LCDs, brightness enhancement films use prismatic structures to direct light along the viewing axes (i.e., normal to the display), which enhances the brightness of the light viewed by the user of the display and which allows the system to use less power to create a desired level of on-axis illumination.
Heretofore, brightness enhancement films were largely provided with parallel prismatic grooves, lenticular lenses or pyramids on the light emitting surface of the films, which change the angle of the film/air interface for light rays exiting the films and cause light incident obliquely at the other surface of the films to be redistributed in a direction more normal to the exit surface of the films. The brightness enhancement films have a light input surface that is smooth, through which light enters from the backlight module.
Heretofore, brightness enhancement films are made up of two layers, including a support base layer and a structured layer. FIG. 1 depicts a sectional structure representative of prior art brightness enhancement films. The brightness enhancement film 100 includes a base layer 102 made of polyethylene terephthalate (PET), and a structured layer 104 of prism structures made of acrylic, which function to redirecting light.
The structured surface of brightness enhancement film 100 is formed after bonding a layer of materials (e.g., an acrylic or polycarbonate layer) to the base layer 102 prior to forming the prism structures in the acrylic layer to form the structured layer 104. The prism structures in the structured layer 104 may be formed using a number of process techniques, including micromachining using hard tools to form master molds or the like for forming the prism structures. The hard tools may be very small diamond tools mounted on CNC (Computer Numeric Control) machines (e.g. turning, milling and ruling/shaping machines), such as known STS (Slow Tool Servo) and FTS (Fast Tool Servo). U.S. Pat. No. 6,581,286, for instance, discloses one of the applications of the FTS for making grooves on an optical film by using a thread cutting method. The tool is mounted onto the machine, to create longitudinal prisms in a plane. The mold may be used to form the structured layer through hot embossing a substrate, and/or through the addition of an ultraviolet curing or thermal setting materials in which the structures are formed.
As shown in FIG. 1, the bottom of the valleys 106 of the prisms in the structured layer 104 is not at the surface of the base layer 102, but spaced at a distance d from the contacting surface of the base layer by acrylic material. In general, the valley bottom thickness d range between 0.3 to 3 micrometers. In order to obtain the bottom thickness, several parameters must be controlled during the curing process to form the structured surface. It has been found that due to inherent limitations during manufacturing processes (including the mold forming process and the structured surface forming process), it is challenging to control a consistent valley bottom thickness d. As noted in the earlier filed U.S. patent application Ser. No. 11/635,802 (which is incorporated by reference herein), unwanted optical cosmetic defects such as ‘chatter’ and/or non-uniformity of the brightness enhancement film are introduced as a result of non-uniformity in the valley bottom thickness in the structured layer. This results in a phenomenon that is easily seen for existing brightness enhancement films, in which repeated dark shades/lines are seen from the planar light source transmitted through the brightness enhancement film.
The valley bottom thickness can be completely missing (i.e., without resin above the base layer, exposing the base layer) at some locations in the structured layer, as a result of defects introduced by the manufacturing processes, as affected by, for example, the manufacturing conditions, environmental specifications and handling processes. The locations with missing valley bottom thickness create optical cosmetic artifacts in the displayed image which are perceivable to a naked eye, such as white spots and white lines in the display image. The artifacts are perceivable because of the high contrast between the exposed base layer and surrounding unexposed areas. For example, a white line defect (see FIG. 1 at 108) is the result of a gap (e.g., 5 μm wide by 620 μm length) of no valley bottom thickness between two prisms, which may be caused by the replication process using the mold. A white spot defect (see FIG. 1 at 109) is the result of a spot (e.g., a 8 μm by 20 μm to 15 μm by 40 μm hole, or even larger 20 μm by 70 μm hole) of no valley bottom thickness at a spot along a valley, which may be caused during releasing the structured layer film from the mold, as resin (e.g., acrylic) was left on the surface of the mold. Other defects may include a row of holes lined up to result in a white line defect in the displayed image viewed by the naked eye.
On the other hand, careless handling may damage the peaks, valleys and/or facets of prisms, for example, by scratches or cutting marks. Other physical conditions and structural deficiencies may also expose the base layer and/or damage prism peaks, valleys and/or facets, for example, cracks and indentations in the structured layer, and foreign particles or materials introduced during molding process but subsequently released from the structured layer.
The push to improve image quality elevates the cosmetic requirements of luminance enhancement optical substrates. A viewer may easily perceive even very small isolated defects. To overcome the optical cosmetic defects noted above, several approaches have been attempted. One solution is to provide a very clean room and use extraordinary care in the manufacturing process, and employ extremely critical quality control procedures. This will significantly decrease throughput, and also highly increase production costs. Another solution is to provide a diffuser to the display. Diffusers with matte structures may mask many of the defects and increase the production yield. This solution, however, increases the components used and enlarges the volume and weight of the display.
What is needed is an optical substrate structure that both enhances brightness and reduces the effects of structural defects on perceived image quality.